Thyroid function after subtotal thyroidectomy for hyperthyroidism

Thyroid hormone replacement: an iatrogenic problem

Thyroid hormone replacement is one of the very few medical treatments devised in the 19th century that still survive. It is safe, very effective and hailed as a major success by patients and clinicians. Currently, it is arguably the most contentious issue in clinical endocrinology. The current controversy and patient disquiet began in the early 1970s, when on theoretical grounds and without proper assessment, the serum thyrotropin (TSH) concentration was adopted as the means of assessing the adequacy of thyroxine replacement. The published literature shows that the serum TSH concentration is a poor indicator of clinical status in patients on thyroxine. The adequacy of thyroxine replacement should be assessed clinically with the serum T3 being measured, when required, to detect over-replacement.

Subclinical hypothyroidism: a review of neuropsychiatric aspects

The authors review current information about the prevalence, causes, course, and consequences of subclinical hypothyroidism. There is evidence that subclinical hypothyroidism may be associated with cognitive dysfunction, mood disturbance, and diminished response to standard psychiatric treatments. Recommendations are presented for the screening, evaluation and treatment of patients in whom subclinical hypothyroidism may be contributing to neuropsychiatric dysfunction.

When to treat mild hypothyroidism

The availability and wide acceptance of TSH assays for primary assessment of thyroid function has led to the recognition that mild thyroid hormone deficiency is characterized by elevation of the serum TSH concentration despite a normal free thyroxine level. Other conditions can also cause isolated serum TSH elevation, and these conditions can be distinguished from mild thyroid failure usually based-on clinical and circumstantial observations alone. Thyroxine treatment of patients with mild hypothyroidism has been shown in most, but not all, clinical trials to lower atherogenic lipid levels and relieve certain somatic and neuropsychiatric symptoms. Such treatment also prevents the progression to overt hypothyroidism, which is particularly likely in patients who are older, who have circulating thyroid autoantibodies, or who have a serum TSH greater than 10 mU/L. After the optimal thyroxine dose has been defined, long-term monitoring of patients with an annual clinical evaluation and serum TSH measurement is appropriate. The high prevalence of mild hypothyroidism, particularly in older women, and its subtle clinical presentation have led some authorities to recommend a low threshold for case-finding or routine population screening for the disorder.

Pattern of recovery of the hypothalamic-pituitary-thyroid axis following radioactive iodine therapy in patients with Graves' disease

PURPOSE

To characterize the time course of recovery of the hypothalamic-pituitary-thyroid (HPT) axis by determining the frequency, onset, duration, and clinical attributes of the central hypothyroid phase following 131I therapy for Graves' disease and to examine whether the central hypothyroid phase is due to direct pituitary thyrotroph suppression or to hypothalamic thyrotropin-releasing hormone (TRH) deficiency.

PATIENTS AND METHODS

Twenty-one hyperthyroid patients with Graves' disease evaluated at a university endocrine clinic and treated with radioactive iodine were prospectively studied. Serial thyroid function levels (serum thyroxine [T4], free thyroxine [free T4], triiodothyronine [T3], and thyroid-stimulating hormone [TSH]) were measured and TRH stimulation tests were performed at 2 to 4 week intervals for all subjects following 131I treatment. None of the patients was treated with thionamides after receiving 131I therapy.

RESULTS

Nineteen (90%) of the patients with Graves' disease experienced a transient central hypothyroid phase defined as the presence of a suppressed or inappropriately normal TSH level despite a low free T4 level following 131I treatment. This phase occurred a mean of 62.8 +/- 5.1 days following 131I treatment, persisted for an average of 24.7 +/- 2.4 days, and was not predictive of eventual treatment outcome. All patients had concordantly low T4 and T3 levels during this period and exhibited a blunted TSH response to TRH compared to 29 euthyroid control subjects, suggesting primary feedback suppression at the level of the pituitary thyrotrophs. The suppressed thyrotrophs required a minimum of 2 weeks to recover once patients became hypothyroid. The length of preexisting hyperthyroidism, basal free T4 elevation, and administered dose of 131I failed to predict the duration of the central hypothyroid phase, although a higher dose of 131I was associated with an earlier onset of central hypothyroidism (r = -.51, P < 0.05).

CONCLUSIONS

Clinicians should be aware of the delay in the recovery of the HPT axis that occurs in the majority of patients with Graves' disease treated with 131I and is manifested by a transient central hypothyroid phase. The blunted TSH response to TRH stimulation during this period suggests that suppression occurs primarily at the level of the pituitary thyrotrophs. The use of sensitive TSH measurements alone to monitor these patients during this period is not helpful and may be misleading.

The management of hyperthyroidism

Although effective treatments for hyperthyroidism are available, none is perfect. Particularly with respect to Graves' disease, what is needed is a therapy directed at modulating the disease process itself rather than merely reducing the synthesis and secretion of thyroid hormones in the hope that the underlying Graves' disease will remit. Greater understanding of the pathogenesis of Graves' disease, resulting from cloning of the thyrotropin receptor and better knowledge of the interactions between these receptors or other thyroid antigens and the immune system, may lead to such treatment. Broad-spectrum immunosuppression, with all its side effects, is not the answer; more focused therapies to inhibit the immune response to specific thyroid antigens may represent the treatment of the future. Meanwhile, radioiodine therapy is the most effective and convenient method of achieving long-term control of hyperthyroidism, although at the cost of hypothyroidism in many patients.

Thyroidectomy for Graves' disease: is hypothyroidism inevitable?

The outcome of 234 patients with Graves' disease treated by subtotal thyroidectomy over a 12-year period is analysed with specific reference to hypothyroidism. Of definite hypothyroid cases, 98% occurred within 2 years. Failure to develop hypothyroidism was statistically related to large remnant size and a large goitre preoperatively. Histological review showed that any degree of lymphocytic infiltration was associated with the development of hypothyroidism (50% vs 22%). Late onset hypothyroidism occurred rarely and is not an inevitable outcome of thyroid surgery in our hands.

The epidemiology of hypothyroidism--an update

Comparison of studies of the prevalence and incidence of hypothyroidism is hampered by differing definitions and population samples. Using a uniform set of diagnostic criteria, the prevalence of previously undiagnosed, spontaneous, overt hypothyroidism in community-based studies has been estimated between 2-4/1000 total population world-wide. If all cases of previously diagnosed hypothyroidism, previous thyroid surgery and radioiodine treatment are included, this prevalence rises to approximately 10/1000, and if subclinical cases are included, then the prevalence is probably over 50/1000 total population. The annual incidence of overt hypothyroidism is between 1-2/1000 for female and around 2/10,000 for males, with individuals having previously elevated TSH and positive circulating thyroid autoantibodies, being particularly at risk. The question of widespread population screening for hypothyroidism is unsettled, but it is probably not cost-effective unless incorporated as part of a screening programme for other conditions such as cervical cancer, or targeted at high risk groups such as post-menopausal women. The combination of serum TSH estimation and a high clinical index of suspicion should detect most patients with thyroid dysfunction, although detailed studies on the use of the more sensitive assays in the detection of both hyper- and hypothyroidism have yet to be published.

The replacement therapy problem in hypothyroidism

There is increasing evidence from studies of heart rate, liver enzyme activity, bone density and urinary sodium excretion that standard replacement therapy doses of thyroxine which suppress TSH secretion are associated with changes in target organ function similar to, but less marked than, those recorded in overt hyperthyroidism. There is also evidence that in subclinical hypothyroidism it is not only the pituitary thyrotroph which recognizes a minor reduction in serum thyroid hormone levels within the normal range. Although there is no proof that slight 'overtreatment' with thyroxine or non-treatment of subclinical hypothyroidism is detrimental to the patient in the long term, the appropriate studies have not been performed. It would seem good clinical practice, however, to treat all grades of thyroid failure and to ensure, if possible, that the dose of thyroxine is adjusted to maintain a normal and detectable TSH level when measured by a sensitive assay system. It must be conceded, however, that with the vagaries of human nature there is always likely to be greater morbidity from patients with hypothyroidism failing to take their medication regularly, than from failure by the medical attendant to make minor adjustments to the dose of thyroxine.

Subclinical hypothyroidism in the elderly: to treat or not to treat?

Patients with subclinical hypothyroidism (SCH) have normal concentrations of thyroid hormone and elevated thyrotropin (TSH) levels. These individuals may experience mild symptoms of hypothyroidism. Such symptoms are nonspecific and also can be associated with aging or nonthyroidal illness. SCH is not uncommon in the elderly, particularly in females with positive thyroid antibodies and in those who have undergone partial thyroidectomy or I131 treatment for Graves' disease. Patients with SCH with markedly increased TSH levels or high-titer thyroid antibodies are at higher risk of progressing to overt hypothyroidism. Management options include observation only, with long-term follow up, or substitution with thyroid hormone. Replacement will prevent the development of overt hypothyroidism when reliable follow-up cannot be assured and may improve subtle, nonspecific symptoms of thyroid hormone deficiency. If a decision in favor of replacement therapy has been made, the dose of thyroid hormone should be increased gradually with the objective of returning the TSH level to normal without inappropriately elevating the serum thyroxine concentration. The patient should be carefully observed to see if hypothyroid symptoms, mental status or cardiac function improve with therapy. Continued administration of thyroid hormone would serve prophylactic purposes even if improvement did not occur.

End-organ responses to thyroxine therapy in subclinical hypothyroidism

We studied variables known to change with thyroid hormone status in 18 patients with subclinical hypothyroidism before and during treatment with thyroxine in a dose sufficient to restore the plasma TSH response to TRH to normal. There was an associated increase in both plasma total T4 and free T4 within the normal range but plasma total T3 and free T3 were unchanged. As a result of thyroxine treatment there was a small but significant increase (P less than 0.05) in left ventricular ejection fraction (LVEF) with maximal exercise but no significant changes in LVEF at rest and moderate exercise, continuously monitored mean sleeping heart rate, day/night ratios of urinary sodium excretion, peripheral nerve conduction velocities, fasting serum triglycerides, total cholesterol (TC), high density lipoproteins (HDL) or TC/HDL ratios. On this evidence we do not consider that thyroxine replacement therapy is indicated in patients with subclinical hypothyroidism.

Late onset hypothyroidism after subtotal thyroidectomy for hyperthyroidism: implications for long term follow-up

A follow-up register has been used in Aberdeen and Dundee to record early and late onset hypothyroidism occurring in a large population of post-thyroidectomy patients treated for hyperthyroidism. In one centre, in a total of 1170 patients, the prevalence of postoperative hypothyroidism, at the time of entry to the register, was 41 per cent. Of these early cases of hypothyroidism 93 per cent occurred within 18 months of operation. Results are presented from a 12-year prospective study of patients treated in two centres, who were euthyroid when entered on the follow-up register. In one centre, based on 683 patients, the 10-year incidence of late onset hypothyroidism estimated by actuarial methods was 7.4 per cent (95 per cent confidence limits, 3.8-11.1); in the other centre with 156 patients the 5-year incidence was 10.8 per cent (95 per cent confidence limits, 3-18.6). The minimum predicted annual incidence is 1 per cent. Large thyroid remnants do not protect some patients against early or late postoperative hypothyroidism but do lead to an increased risk of recurrent hyperthyroidism. Hypothyroidism after subtotal thyroidectomy for hyperthyroidism shows a bimodal pattern and this study emphasizes the need to maintain life-long follow-up.

Long term follow-up of hyperthyroid patients treated by subtotal thyroidectomy

The early and long term complications of subtotal thyroidectomy in 306 hyperthyroid patients (multinodular goitre and diffuse hyperplasia) followed for up to 30 yr are reviewed. There were no perioperative deaths. Sixteen patients (5.2 per cent) had transient symptomatic hypocalcaemia, while 9 (2.9 per cent) had permanent hypocalcaemia. Permanent unilateral vocal cord paralysis occurred in 11 (3.6 per cent) patients (1.8 per cent of nerves at risk). Cumulative per cent (+/- s.e.m.) relapse and hypothyroid rates at 30 yr (life-table analysis) were 15.6 +/- 2.4 per cent and 20.5 +/- 2.1 per cent, respectively. Lifelong follow-up of post-thyroidectomy patients is mandatory because of the risk of relapse of hypothyroidism.

What is the significance of raised plasma TSH levels after thyroid surgery?

Thyroid function was assessed prospectively for five years in 73 patients treated surgically for Graves's disease. No patient developed hypothyroidism after the sixth postoperative month, despite the presence of raised plasma TSH levels in 70 per cent of euthyroid patients at the end of the first year. Indeed, in those with evidence of temporary hypothyroidism (low T4, raised TSh at 3 months but normalisation of T4 at 6 months) plasma TSH continued to fall for up to three years. The majority (6) of patients developing recurrent hyperthyroidism did so within the first year, but in one of three patients who relapsed subsequently, plasma TSH had been elevated at one year. Plasma TSH cannot be used to predict thyroid status following surgery for Graves' disease. Although regular review remains necessary, it should not include measurement of TSH unless this is needed to confirm the validity of a low thyroxine level after the third postoperative month and before starting permanent replacement therapy.

Results following surgical treatment of hyperthyroidism

In the years 1965-1978 1,222 patients with different types of thyrotoxicosis underwent surgical treatment at the 1st Department of Surgery, University of Vienna. Wherever possible a sparing selective surgical approach was considered preferable: autonomous adenoma (45%) enucleation resection or subtotal uni-lateral resection; multinodular toxic goiter (36%) and Graves disease (5%) uni- or bilateral subtotal resection. The remaining 5% were rather rare types of goiter (recurrent goiter, thyroiditis, adenocarcinoma). Overall mortality due was 0.7%. One-hundred and seven patients (8.76%) were over 70 years old at the time of the operation. Post-operative death occurred in the group of patients with toxic adenomas (2.7%). Four-hundred and ninety-five patients were followed up from 3-13 years postoperatively: the rate of recurrent thyrotoxicosis was 4.4%, 4.8% of the patients with hypothyroidism. In this paper the significance of the rapid effect of surgery in cases of hyperthyroidism is discussed and the results are compared with findings in other studies.

Post-thyroidectomy thyrotoxicosis

94 patients with postoperative recurrent hyperthyroidism were evaluated for duration of remission, goitre size, and response to radio-iodine (131I). 6 patients required 131I therapy within twelve months of operation--5 had large remnants because of inadequate surgery. 57% of patients relapsed within 5 years, but 16% relapsed after 20 years and 8% after more than 30 years. Estimated goitre weights ranged from 4 g to 65 g, and goitre size was unrelated to the duration of remission. All patients were treated with 131I. 23% of the patients became hypothyroid in the first postoperative year and 10% in the second year. The results indicate that postoperative thyrotoxicosis can recur decades after operation. Operation seems to sensitise the thyroid to the early effects of radiation by 131I.

Thyroid dysfunction following external irradiation to the neck for Hodgkin's disease in childhood

Thyroid function was studied in 32 patients who had received neck irradiation during childhood for Hodgkin's disease. All except one patient received a dose of 2500-3000 rad over a period of 19-25 days. In 12 patients lymphangiography was performed. Clinically all patients were considered euthyroid. One had a thyroid swelling which was cystic in nature. Five (16%) patients were biochemically hypothyroid, 17 (53%) were euthyroid with an elevated basal serum TSH concentration and a further seven (22%) were euthyroid with a normal basal serum TSH level but an augmented thyroid-stimulating hormone (TSH) response to thyrotrophin-releasing hormone (TRH). Only three (9%) patients had completely normal thyroid function tests. The basal serum TSH concentration and the peak serum TSH response to TRH were significantly greater in the patients who received neck irradiation and lymphangiography than in those who received neck irradiation alone. In addition the free thyroxine index decreased significantly as the time interval between treatment and study increased in the lymphangiography group. These data demonstrate that the normal thyroid gland is vulnerable to the damaging effects of external irradiation, and that the combination of neck irradiation and lymphangiography is more likely to result in thyroid dysfunction than is neck irradiation alone. Furthermore, in view of the deterioration in thyroid function with time, periodic clinical and biochemical assessment of thyroid function is clearly indicated.

Hypothyroidism following partial thyroidectomy for thyrotoxicosis and its relationship to thyroid remnant size

One hundred and twenty-two patients were reviewed 1-7 years after partial thyroidectomy for thyrotoxicosis by two surgeons who had left thyroid remnants of different size. There was no significant difference in the prevalence of hypothyroidism or in the serum levels of thyroxine, tri-iodothyronine or thyroid-stimulating hormone between the two groups of patients. The overall prevalence of hypothyroidism was 16 per cent.

A long-term follow up of patients with autoimmune thyroid disease

A survey in a general practice in the North-East of England in 1963 detected thyroglobulin antibodies in 16.2% of women and 4.3% of men. High titres of antibodies were found in 4.6% of women and 1.6% of men. Forty six subjects with thyroglobulin antibodies (from an original total of fifty-two) were studied in 1972 and forty of these were studied further in 1975. These subjects were compared with a group of age- and sex-matched controls from the original survey. Three of the subjects had developed overt hypothyroidism by 1975 and a raised serum thyroid-stimulating hormone (TSH) concentration was found more frequently in euthyroid subjects peviously found to be antibody positive. There was a striking difference in the antibody studies in that only 26% of the previously antibody positive subjects had thyroglobulin antibodies in 1972 and 30% in 1975. A raised serum TSH concentration was found to correlate with cytoplasmic a-tibodies and particularly with the combination of cytoplasmic and thyroglobulin antibodies.

Serum thyrotrophin concentration: an unreliable test for detection of early hypothyroidism after thyroidectomy

Three groups of patients who had undergone subtotal thyroidectomy for Graves's disease, toxic multinodular goitre, or euthyroid multinodular goitre 12 to 15 years before and in whom a normal serum thyroxine (T-4) level was found were each divided into two subgroups on the basis of a normal or a raised serum thyrotrophin concentration. There was no difference in mean serum T-4 concentration between patients with normal and those with raised serum thyrotrophin concentrations, and the values were similar to the mean T-4 values of the normal population. The mean serum triiodothyronine values of all groups were higher than normal, but the mean values of the groups with a normal and a raised serum thyrotrophin were similar. After thyroidectomy a mildly raised serum thyrotrophin does not in itself indicate the presence of hypothyroidism.

Thyroid function in the long-term follow-up of patients treated with iodine-131 for thyrotoxicosis

In February, 1972, 58% of patients euthyroid after iodine-131 therapy for thyrotoxicosis between 1954 and 1966 had a raised plasma thyroid-stimulating-hormone (T.S.H.) (greater than 7-4 mU/l) and 42% a normal T.S.H. level. A group of 69 of the euthyroid patients with a raised plasma T.S.H. (25-0 +/- 2-0 mU/l) in 1972 was re-examined annually for three years. There was no apparent change in the mean plasma T.S.H. level between 1972 and 1975 in the patients remaining euthyroid, but overt hypothyroidism developed in 3 patients in 1973, in a further 3 patients in 1974, and in 1 patient in 1975. In contrast, none of a group of 61 patients, euthyroid with a normal plasma T.S.H. (4-0 +/- 0-2 mU/l) in 1972, developed overt hypothyroidism over the next three years, although slightly raised T.S.H. levels were recorded in 3 patients in 1974 and in a further 6 patients in 1975. Both the mean serum T-4 and T-3 in the euthyroid patients with a raised plasma T.S.H. were significantly lower, but still in the respective normal ranges, than those in the euthyroid patients with a normal plasma T.S.H. No significant difference in the fasting serum-cholesterol or triglyceride levels could be demonstrated between the two groups. Since no patient with a normal plasma T.S.H. after iodine-131 treatment for thyrotoxicosis six to eighteen years earlier developed overt hypothyroidism over a three-year period, the follow-up of such patients need not be so frequent as that of similarly treated euthyroid patients with a raised plasma T.S.H. in whom overt hypothyroidism develops at the rate of 2-5% per year.

Whither thyrotoxicosis?

For 30 years the thyrotoxic patient has been subjected to a plurality of treatments by surgery, radio-iodine and long term anti-thyroid drugs. These therapies have been accepted as complementary to the needs of the individual patient, without regard for long term results or the economic situation as it affects both patient and hospital services. In the context of surgical treatment which is now available, it is suggested that the advantages of operation over other therapies presage a reversion to surgery as the treatment of choice.